The present invention relates to a photomask used for manufacture of semiconductor devices, a method of manufacturing the photomask, a method of forming a pattern using the photomask, a method of manufacturing semiconductor devices using such a pattern formation method, and a mask pattern design system.
A lithography technique of printing a pattern depicted on a mask to be exposed onto a substrate has been extensively used for forming a pattern upon manufacture of a semiconductor device or liquid crystal panel. For this pattern printing, a reduction projection exposure system for printing a pattern depicted on a mask at a specified reduction ratio has been generally used.
To meet the recent tendency of making fine the size of a pattern, the projection exposure system has been required to be enhanced in its resolution. In general, as the numerical aperture of a projection lens (NA) becomes larger, or as the wavelength of exposure light becomes shorter, the resolution of the system becomes higher. However, an increase in the NA reduces the depth of focus upon pattern printing, and the shortening of wavelength of light is variously restricted from the viewpoint of a light source, optical material and resist material.
An attempt has been made to print a finer pattern at a resolution exceeding the conventional limit using the existing projection exposure system. Unexamined Japanese Patent Publication No. SHO 57-62052 discloses a method of providing a transparent film (phase shifter) for inverting the phase angle of light on a specified light passing portion on a mask, thereby significantly improving the resolution for a plurality of periodic patterns.
A technique of automatically arranging the above-described phase shifter has been reported in VLSI Symposium (1991), and has been described in Digest of Technical Papers , pp. 95-96! titled "Automatic Pattern Generation System For Phase Shifting Mask". The similar techniques have been described in Digest of Papers Microprocess '93, pp. 50-51! titled "Algorithm for Phase Shift Mask Design with Priority on Shift Placement", and in the same paper (pp. 52-53) titled "A CAD System for Designing Phase-Shifting Mask". A technique for inputting a desired shape of a projected image for determining a mask pattern has been described in IEEE Transaction on Semiconductor Manufacturing, Vol. 5, No. 2 (1992), pp. 138-152! titled "Binary and Phase Shifting Mask Design for Optical Lithography".
On the other hand, U.S. Pat. No. 4,360,586 has disclosed a photomask of controlling the intensity and phase angle of X-ray or light passing the mask. The similar photomask has been disclosed in Unexamined Japanese Patent Publication No. HEI 04-1368540.
In each of these photomasks, the peripheral portion of a single transparent pattern becomes semitransparent, that is, a usual opaque film of a photomask is made semitransparent, and the phase angle of a slight amount of a light beam passing though the semitransparent portion is inverted relative to the phase angle of a light beam passing through the transparent pattern. Since the light beam passing through the semitransparent portion is inverted in phase angle relative to the light beam passing through the transparent pattern as a main pattern, it is inverted at the boundary therebetween, and thereby the light intensity becomes close to zero at the boundary. As a result, the ratio between the intensity of the light beam passing through the transparent pattern and the light intensity at the boundary is relatively increased Thus, in this photomask, a light intensity distribution being high in contrast as compared with the usual Cr mask can be obtained, and the depth of focus upon pattern formation is nearly doubled. The mask structure can be easily realized only by replacing the conventional Cr mask with a semitransparent film having a phase inversion function. Hereinafter, such a mask is referred to as a halftone phase shift mask (attenuate phase shift mask).
A technique of providing auxiliary patterns at both the ends of each of a plurality of periodic patterns and non-periodic patterns of the similar halftone phase shift mask for improving the shape of a projected image has been described in Digest of Papers MicroProcess '93, pp. 44-45) titled "Resolution Improvement using Auxiliary Pattern Groups in Oblique Illumination Lithography".
The prior art described in Unexamined Japanese Patent Publication No. SHO 57-62052 can significantly improve the resolution for a plurality of periodic patterns; however, a method of arranging a phase shifter for a usual non-periodic pattern has not been described.
The prior art described in Digest of Technical Papers, pp. 95-96!, Digest of Papers MicroProcess '93, pp. 50-51 and pp. 52-53! and IEEE Transaction on Semiconductor Manufacturing, Vol. 5, No. 2 (1992), pp. 138-152! relate to a method of automatically arranging a phase shifter on a light passing portion on a mask.
In the halftone phase shift mask, since a slight amount of passing light is present in an area to be shielded, the passing light is emphasized by an interference effect when the patterns are close to each other, and consequently, there arises a problem that a projected image having a light intensity level that is not negligible is formed on a portion where a pattern should not be present.
Even in the usual Cr mask, there arises the similar problem that a projected image is formed on a portion where a pattern should not be present by an interference effect, although the interference effect is lower than in the halftone phase shift mask.
The prior art reference do not describe a method of solving the above-described problem due to the interference effect.
The prior art described in U.S. Pat. No. 4,360,586 and Unexamined Japanese Patent Publication No. HEI 04-136854 fail to solve the problem of reducing the depth of focus of a fine pattern. Therefore, they present problems that upon manufacture of a fine element, a production yield is reduced due to a resolution failure, and that a projected image is formed on a portion where a pattern should not be present.
The prior art described in Digest of Papers MicroProcess '93, pp. 44-45! uses an annular illumination as a light source exposure and no description is made for a system with a low annular condition .sigma..